Radiofrequency (RF) probes employed in electrosurgical procedures are generally divided into two categories: monopolar devices and bipolar devices. In monopolar electrosurgical devices, the RF current generally flows from an exposed active electrode through the patient's body, to a passive or return current electrode that is externally attached to a suitable location on the patient's skin. In bipolar electrosurgical device, both the active and the return current electrodes are exposed and are typically in close proximity. The RF current flows from the active electrode to the return electrode through the tissue. Thus, in contrast with the monopolar electrosurgical devices, the return current path for a bipolar device does not pass through the patient's body except for close proximity to the tip of the electrode.
Electrosurgery is the intentional passage of high frequency current through tissue to achieve a controlled surgical effect. This can be accomplished in an oxygen rich, an inert gas, or a conductive fluid media environment. Arthroscopic tissue ablation is performed in a conductive fluid environment, such as inside of a joint or body cavity filled with, for instance, normalized saline solution, and differs from that described previously in that current is conducted from the active electrode through the fluid to the return electrode. In the case of a monopolar device, the current flows through the patient to the return electrode in the manner previously described. In the case of bipolar devices operating in a conductive fluid environment, the return electrode is not in contact with tissue, but rather is submerged in the conductive fluid in the proximity of the active electrode. Current flow is from the active electrode through the conductive liquid and surrounding tissues to the return electrode of the bipolar device. Whether an electrode is monopolar or bipolar, current flows from all uninsulated surfaces of the active electrode to the return electrode anytime that the probe is energized. This is in contrast to conventional surgery (also called “open surgery”) in which current flows only through electrode surfaces in contact with the patient's tissue.
During the past several years, specialized arthroscopic electrosurgical probes also called ablators have been developed for arthroscopic surgery. Ablators differ from the conventional arthroscopic electrosurgical probes in that they are designed for the bulk removal of tissue by vaporization, rather than by cutting the tissue or coagulating the bleeding vessels. This way, during ablation, volumes of tissue are vaporized rather then discretely cut out and removed from the surgical site. Aspiration ports in the ablator are often provided to remove ablated tissue and debris.
The power requirements of ablators are generally higher than those of other arthroscopic probes. The efficiency of the probe design and the characteristics of the radio frequency (RF) power supplied to the probe also affect the amount of power required for ablation. For example, probes with inefficient designs and/or powered by RF energy with poorly suited characteristics will require higher powers levels than those with efficient designs and appropriate generators. Probes used in electrosurgery have relatively large area of metallic electrode, which is the active area of the probe. Large electrode area decreases the probe impedance and, therefore, increases the RF power required for proper operation. The shape of the dielectric insulator and of the probe tip can significantly affect ablation. By properly shaping the insulator and the electrode tip, the threshold power can be substantially decreased.
A recent improvement to ablation electrodes is the addition of aspiration to remove bubbles and debris from the surgical site. During electrosurgery in a conductive fluid environment, tissue is vaporized, thereby producing steam bubbles which may obscure the view of the surgeon or displace saline from the area of the intra-articular space which the surgeon wishes to affect. In the case of ablation (bulk vaporization of tissue), the number and volume of bubbles produced is even greater than when using other electrodes since fluid is continually boiling at the active electrode during use. Ideally, flow through the joint carries these bubbles away; however, in certain procedures this flow is frequently insufficient to remove all of the bubbles. Aspiration removes some bubbles as they are formed by the ablation process, and others after they have collected in pockets within the joint. The aspiration portal is connected to an external vacuum source which provides suction for bubble evacuation.
Aspirating ablators are divided into two categories according to their level of flow. High-flow ablators have an aspiration tube, the axis of which is coaxial with the axis of the ablator rod or tube, which draws in bubbles and fluid through its distal opening and/or openings cut into the tube wall near its distal tip. High-flow ablators may decrease the average joint fluid temperature by removing heated saline (waste heat since it is an undesirable biproduct of the process) from the general area in which ablation is occurring. The effectiveness of the aspiration, both for removal of bubbles and for removal of waste heat, will be affected by the distance between the opening through which aspiration is accomplished and the active electrode. The distal tip of the aspiration tube is generally several millimeters distant proximally from the active electrode so as to not to obstruct the surgeon's view of the electrode during use. Decreasing this distance is desirable since doing so will increase the effectiveness of the aspiration. However, this must be accomplished without limiting the surgeon's view or decreasing the ablator's ability to access certain structures during use.
Low-flow ablators are those which aspirate bubbles and fluid through gaps in the ablating surfaces of the active electrode and convey them from the surgical site via means in the elongated distal portion of the device. Current low-flow ablators require increased power to operate as effectively as a nonaspirating or high-flow aspirating ablators because the low-flow aspiration draws hot saline from the active site of a thermal process. In the case of low-flow ablators, the heat removed is necessary process heat rather than the waste heat removed by high-flow ablators. Because of this, aspirating ablators of the low-flow type generally require higher power levels to operate than other ablators thereby generating more waste heat and increasing undesirable heating of the fluid within the joint.
Each of these types of aspirating ablation electrodes has its drawbacks. In the case of high-flow aspirating ablators, the aspiration tube increases the diameter of the device thereby necessitating the use of larger cannulae which, in turn, results in an increase in wound size and often an increase in patient pain and recovery time. In the case of low-flow aspirating ablators, the devices decrease the efficiency of the probes since process heat is removed from a thermal process. This decreased efficiency results in decreased rates of tissue removal for a given power level. This results in increased procedure times or necessitates the use of higher power levels to achieve satisfactory tissue removal rates. High power levels are undesirable as they cause increased heating of the fluid at the site and thereby increase the likelihood of thermal injury to the patient.
Accordingly, it is desirable to provide an electrosurgical probe of high efficiency and high impedance with an improved design of the aspiration port, and which is capable of conferring high ablation rates at low RF power levels. An electrosurgical ablation electrode, which aspirates through the ablating portion of the active electrode and has increased ablation efficiency as compared to existing ablation electrodes which aspirate through the active electrode, is also desirable.